Orthogonal Code Division Multiplexing for Twisted Pair Channels

ABSTRACT

A plurality of data signals are separated into parallel bit streams with each parallel stream having a bandwidth characteristic such that the combined cumulative effect of all the individual bandwidths produces a spectral characteristic of the data signals that match the spectral high speed data characteristic of a twisted pair

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.10/832,762 filed in the United States Patent Office on Apr. 27, 2004,which application is a continuation of U.S. application Ser. No.09/413,162 filed Oct. 6, 1999 which now bears U.S. Pat. No. 6,711,121.

FIELD OF THE INVENTION

This invention relates to high-speed data transmission over a twistedpair local access connection. It is specifically concerned with matchingof power spectra density of a high rate data channel with a transferfunction of a twisted pair.

BACKGROUND OF THE INVENTION

High-speed data transmission over a twisted pair (i.e., subscriber localaccess line) such as HDSL, as it is presently carried out, usestransmission procedures full of complexity. Such complexities includemulticarrier modulation, multitone transmission and orthogonal frequencydivision. These processes are all susceptible to external interferenceand to crosstalk.

Transmission of signal s(t) in a Twisted pair is subject to varioustypes of interference including near end crosstalk (NEXT), which affectshigh speed data transmission. Multiple channels may be transmitted overa single twisted pair, but may interfere with one another. Twisted pairchannels must be substantially orthogonal to one another in order tolimit interference between channels.

A functional diagram of these interferences is shown in the FIG. 1A,where an input lead 101 represents application of the data signal s(t)to the twisted pair. The twisted pair may be characterized by a transferfunction that is related to the absolute square of the functional valueH_(c)(f), which represents an attenuation characteristic at the twistedpair, and is proportional to √{square root over (f)}. In addition, thereis significant interference from NEXT, represented by H_(a)(f). Far endcrosstalk, FEXT, is relatively very low compared to NEXT and is notincluded in the model.

The input and the NEXT are applied to mixer 107 that represents theinteraction of the signals. The output on lead 109 represents theamalgam of the data signal plus the interference signals.

The effect of this interference of the twisted wire attenuation ofsignals is shown by the graph of FIG. 1B where coordinate axis 100 ofthe log of signal frequency is plotted against a coordinate axis 102representing attenuation in dB.

Curve 111 represents the attenuation of the data signal as function offrequency. Curve 113 represents the increase of NEXT as frequencyincreases. It is apparent that as the data signal attenuates withincreasing frequency, and the interference signal increases with theincreasing frequency. NEXT is a dominant portion of this interference.Other forms of interference include narrow band radio interference. Allthese interferences contribute to the frequency limits of the twistedpair. In a typical instance 95% of twisted wire capacity is below 10 kHZand 60% of capacity is below 40 kHZ.

SUMMARY OF THE INVENTION

A plurality of data signals are separated into parallel bit streams,with each parallel stream having a bandwidth characteristic such thatthe combined cumulative effect of all the signals with individualbandwidths produce spectral characteristics of the data signals that canaccommodate and emulate the spectral high speed data transmissioncharacteristic of a twisted pair.

In a particular embodiment parallel data streams resulting from a serialto parallel conversion of an input data stream and multiplexing into anumber of parallel symbol streams, including data transmitted atdifferent data rates are each individually spread to a differentbandwidth so that the combined effect of the selected bandwidths has aspectral distribution resembling the spectral transmissioncharacteristic of a twisted pair.

A proposed embodiment based on Orthogonal Code Division Multiplexing(OCDM) performs two essential steps: (1) performing a serial to parallelconversion of a serial input into a plurality of orthogonal serialstreams each having different chip rates, and (2) loading of each of theparalleled streams by adaptive modulation. These two steps are combinedwith spreading code generation techniques to achieve high-speed datatransfer over twisted pair.

Each individual paralleled stream is spread to a different bandwidth,with the narrowest bandwidth having the highest modulation loading andthe broadest bandwidth having the lowest modulation loading. Theselected modulation technique in one embodiment is pulse amplitudemodulation with no carrier. Adaptive loading of the signal streams(loading refers to a matrix dimensionality of a modulating signal) isselected and applied to distribute the power spectra so that it matchesthe twisted pair transfer function. Each stream may represent differentservices.

Fundamentally the concepts of the OCDM scheme are a serial-to-parallelconversion (i.e., multi-codes) and overspreading (i.e., spectralmatching). An input data stream is serial-to-parallel converted toparallel branches. Each branch is transmitted with spreading orthogonalcodes applied. The number of parallel branches N is equal to thespreading gain of these orthogonal codes. Spectral matching to thechannel is preceded by an overspreading of the chips of the spreadingorthogonal code. Overspreading is recursive with a (+,−) pattern.Different powers are assigned to different levels. Different levels ofoverspreading are orthogonal to each other. Serial-to-parallel codes maybe reused in spreading and overspreading in different channels ofoverspreading according to a spectral matching desired.

An advantage of the coding scheme is that it is effective in rejectingradio interference even with unshielded twisted pair.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1A is a functional block schematic of a twisted pair channelsubject to interference including near end crosstalk (NEXT);

FIG. 1B is a graph of the frequency and attenuation response of thetwisted pair channel functionally shown in FIG. 1A;

FIG. 2A is a block schematic of an orthogonal coded transmittingprocessing circuit for optimizing data rates and applying signals to atwisted pair channel;

FIG. 2B presents graphs that compare input and output rates of theprocessing circuit of FIG. 2A;

FIG. 3 is a block schematic a receiving terminal for receiving OCDMtransmissions from a transmitter such as shown in FIG. 2A;

FIG. 4 is a block schematic of a service multiplexing circuit usingorthogonal code multiplexing for providing T1, ISDN and POTS servicesover a twisted pair channel;

FIG. 5 is a block schematic of a generalized OCDM circuit using adaptivemodulation to achieve a spectral density to match a twisted pairtransfer function;

FIG. 6A is a block schematic of a particular example of an OCDM withadaptive loading of the modulation process to achieve a match of thetransmitted data power spectra characteristic with that of the twistedpair;

FIG. 6B is a block schematic of a particular example of an OCDM withadaptive loading of the modulation process;

FIG. 7 is a graph of power spectral density for the OCDMs using adaptiveloading of modulation and compared with the power spectra characteristicof a twisted pair; and

FIG. 8 is a graph of spectral matching OCDM with a twisted pairspectrum.

DETAILED DESCRIPTION

In accord with the principles disclosed herein, data transmissions aredistributed by frequency so that the highest data load is at lowerfrequencies, where the capacity is high, and the lowest data load istransmitted at high frequencies. Orthogonal Code Division Multiplexing(OCDM) is utilized to achieve this objective. Multiplexing of existinglow bit rate services are multiplexed with high data rate T1 service. Ina particular embodiment the OCDM is a carrierless baseband system withadaptive pulse amplitude modulation (PAM) loading.

The drawing and text uses various symbols in the various descriptions.These symbols are as follows:

W: the overall Twisted Pair Channel (TPC) useful bandwidth;

R_(c): The basic chip rate;

M: The total number of baseband sub-rate OCDM components (groups ofbranches) set to match power spectral density of the channel;

R_(c), 2R_(c), 4R_(c), . . . 2^(M−1)R_(c) are chip rates used by Mgroups of OCDM with the relationship between basic chip rate and totalOCDM bandwidth as:

$R_{c} = \frac{2\; W}{2^{M - 1}}$

N: processing gain associated with the basic chip rate; also the numberof OCDM parallel data streams and of orthogonal codes used.

N₁, N₂, . . . N_(M): numbering of parallel data streams out of N used ineach branch.

N_(m) corresponds to a group of branches m whose signal has a chip rateand bandwidth 2 ^((m−1)) for m=1, 2, . . . , M

L_(m) is a measure of the order of data modulation (i.e., loading):represents the total number of symbols is used;

I_(m)=log₂L_(m) represents the number of bits per L_(m)-PAN symbol.L_(m)-PAN is used by all N_(M) data streams or branch m.

FIG. 2A shows the circuitry used for conditioning signals fortransmission over a twisted pair. An incoming symbol stream of rate R isapplied to lead 201. Its output, at rate Rb, is applied to a M-PAM pulseamplitude modulator 205. Modulator 205 maps the FEC output into k-bitblocks, or symbols, and converts each symbol into a signal level,resulting in a stream of pulses each having a level ranging between 0and M−1, where M=2^(k). The M-PAM modulator in this embodiment operatesat base band and is carrierless.

The M-PAM pulses, having the rate R_(s), are demultiplexed bydemultiplexer 207 into N paralleled streams, each having a rate R_(s)/N,with the signal content of each stream being spread by one of Northogonal codes H_(i) where i=1, . . . N (developed in block 221)having a rate R_(s) in mixers 215-1, . . . , 215-N. Codes H_(i) spreadall of the parallel streams to a common rate R_(s). The resultingparalleled streams 209-N are summed in summing circuitry 211. Thecombined signal is cover coded, in mixer 219, with a PN code (providedby block 223) and applied to a pulse-shaping filter 213 to shape outputpulses so that its power spectral density (PSD) approximates thatobtained from a water pouring solution. Water pouring is a concept usedin the science of information theory.

If all data sources are linked or co-located, the system does notrequire system synchronization of the symbol streams. If the symbolsources arise from widely separated source locations, the twisted pairchannels (TPC) will be asynchronous, leading to increased interference.Interference between channels may still be reduced by application of thePN cover code. Interference is reduced by a factor of R_(o)/N whereR_(o) is the symbol rate in another TPC (i.e., twisted pair channels).

The graphical displays of FIG. 2B illustrate typical differing coderates as a step function 251 showing different code rates 1 through 8.These code rates are demultiplexed into 8 parallel symbol streams 253 ata rate R_(s).

It may be noted that the modulating codes H_(m) are typicallymulti-dimensional (i.e., matrices of multiple dimensions). In theillustrative embodiment overspreading is performed using all availableorthogonal codes so that usage of the codes is complete. This improvesspectral matching with the twisted pair.

Signal reception requires an OCDM receiver to recover the individualsignals at a termination of the twisted pair. In an OCDM receiver shownin FIG. 3 the received OCDM signal at lead 301 is covered by a applyinga PN code g_(i) at mixer 303 in order to achieve a desired power spectradistribution of the received signal. The covered signal R_(s) is appliedto mixers 307-1, . . . , 307-N. The mixers in response to an applieddespreading code and in combination with subsequent associatedaccumulators 309-1, . . . , 309-N recover the original CDMA signal. Theoutput R_(s)/N of each accumulator is applied to a multiplexer 311,which recovers the input signal R_(s), and the signal is subsequentlydemodulated in demodulator 313. The demodulated signal is applied to adecoder 317, which decodes the FEC coding and a final output is appliedto output lead 315.

The OCDM system provides a method of code multiplexing a number ofservices for the twisted pair channel. Transmission circuitry suitablefor this application is shown schematically in FIG. 4. As shown T1,ISDN, and POTS services are applied to an OCDM transmitter. T1 having asignal rate R_(s1) is applied to a demultiplexer 411 and demultiplexedinto N₁ parallel streams at rate R_(s1)/N₁. R_(s2) is an ISDN signaldemultiplexed by demultiplexer 413 and divided into parallel streamshaving the rate R_(s2)/N₂. POTs signals are converted from analog todigital having a race R_(s3). All of the parallel streams are spread byN orthogonal codes with rate R_(s), by mixers 419-N and applied tosumming circuit 421, which combines all the streams into one stream. APN cover code of rate R_(s) is mixed, in mixer 431, with the summeroutput and the output is filtered with a power spectrum filter 415 toobtain a desired pulse shape. In this manner the provision of multipleservices may be provided by code multiplexing.

System efficiency may be greatly enhanced by adaptive loading in whichthe dimension of modulation is changed, for various individual streams,to control the symbol signal bandwidth within a particular twisted pairchannel. The input source data is divided into a number of parallelstreams. Each stream is loaded with a different order of modulation.Each of the parallel streams is further demultiplexed into substreamsfollowing modulation which are spread and applied to another demultiplexstep.

A fundamental circuit for distributing bandwidth of various portions ofa serial stream to match a twisted pair transfer function is shown inthe FIG. 5. A stream is introduced at FEC 501 serial to parallelconverter 503 into parallel streams of rates R₁, R₂ and ending withR_(m). Each of these streams is adaptively modulated (loaded) in themodulators 511-1, . . . , 511-N to achieve a particular bandwidthcharacteristic. The paralleled streams are further divided intoparalleled substreams R₁/N₁ up to R_(m)/N_(m), each of the substreams ismixed with Walsh codes W_(N) _(M) , and the multiple outputs ofmodulators 511-1, . . . , 511-N that are thus augmented are summed insumming circuits 521-1, . . . , 521-N, respectively. A first output onlead 525 is at rate R_(c). The output of summing circuit 521-2 is mixedwith the orthogonal code H₁ to achieve an output 2R_(c). The summingcircuit 521-N output is multiply mixed with a series of orthogonal codesH₁, H₂, . . . H_(M). This combination of mixing and modulation achievesa frequency distribution of the data rates of the incoming stream.

The substreams are spread by orthogonal codes with the code spreadingcreating bandwidths of different resulting power spectral densities thatallow mixing and matching to provide a power spectra characteristicmatching that of the twisted pair transfer function. A typical result isas shown in the graph of FIG. 7 wherein the individual power spectradistribution is shown for three data streams for rates of R_(c), 2R_(c),4R_(c) and MR_(c) are shown by curves 701,702,703 and 704, respectively.The shape of each of these data streams is due to the pulse-shapingfilter. All of the overlapping streams are orthogonal to one another.The sum of powers in each band is adjusted to achieve the desiredcumulative effect of all streams in order to match the twisted pairpower spectral characteristics. The modulation load is higher at thelower frequencies, which include the most useful channel capacity. It isreadily apparent from the FIG. 7 graph that the cumulative effect of thedifferent bands, created by adaptive modulation loading, R_(c), 2R_(c),and 4R_(c) presents a cumulative component which approximates thetransfer characteristic of the twisted pair as indicated by the line705.

Distribution of bandwidth by adaptive modulation is performed bytransmitters, such as shown in FIG. 6A. FEC coded signals R_(s), fromFEC 602 are applied to a demultiplexer 603 generating outputsR_(x)=R_(s)/3, R_(y)=R_(s)/4 and R_(y)=R_(s)/4. These signals aremodulated in 256-PAM, 64-PAM and 8-PAM modulators 604, 606 and 608,respectively. Each modulated stream is again demultiplexed by one of thedemultiplexers 613, 615 and 617. Demultiplexer 613 generates an outputof parallel streams having a rate of R₁/24. Demultiplexer 615 generatesR₂/16 and Demultiplexer 617 generates an output of R₃/32.

All the demultiplexed signals are spread by mixers 623-1, . . . , 623-N,625-1, . . . , 625-N and 627-1, . . . , 627-N and applied to summers633, 635 and 637, respectively. Their respective outputs are cover codedby PN codes g_(i) in mixers 643, 645 and 647. The output of mixer 643 isapplied to power spectra filter 671, output of mixer 645 is spread withcode W₁ in mixer 655 and applied to power spectra filter 672, and theoutput of mixer 647 is spread with coder W_(g). The outputs of powerspectra filters 671, 672 and 673 form the desired loading and bandwidthdistribution at outputs 663, 665 and 667, respectively.

A similar circuit in FIG. 6B is provided with additional mixers 675, 676and 677 in the output to permit added overspreading of the output bycodes g₂ and g₃. Overspreading as used herein means a re-spreading of agiven spread signal of rate R with an orthogonal code having a ratewhich is an even integer multiple of the spread signal being overspread.

The adaptive loading produces a bandwidth distribution in which narrowerspreading bandwidths have a higher order modulation and wider spreadingbandwidths have a lower order modulation. As shown in the FIG. 7 graphthe spreading bands have overlapping regions but the maintenance oforthogonality prevents significant interference. This approach satisfiesthe requirements needed for use of the water-pouring scheme whichpermits maximum utilization of the TPC capacity. Three data streams areshown in the graph and these bands are due to power spectra filtering.In the first frequency band (0,R_(c)) three data spectra overlap. Asecond frequency band (R_(c), 2R_(c)) has two overlapping regions. Bycontrol of the sum of powers in each band the powers in each band may beregulated so that the overall sum of all the bands approximates thetransfer characteristic of the twisted wire pair according to waterpouring solutions. Water pouring allows matching of spectral energy withfrequency. Both transmit power and pulse shape may be adjusted in eachstream and the number of streams delineated may also be adjusted.Modulation loading is highest at lower frequencies where the mostchannel capacity is located.

The graph of spectral matching versus frequency is shown in FIG. 8. Asshown, the curves for the different groups are almost coincident withone another.

1. A method for developing a signal adapted for transmission over atwisted pair wire comprising the steps of: accepting an incoming dataflow from a plurality of sources where each of the sources S_(i), i=1,2, . . . K, provides symbols at rate R_(i) that is a multiple of rateR_(s); splitting each flow S_(i) into a number of parallel streams suchthat each of the parallel streams has a symbols rate of R_(s), forming atotal number of N streams from the splitting of all flows S_(i);spreading each of the N streams with a different spreading code from anorthogonal code set on N codes; combining the spread signals; modifyingoutput signal that results from said combining by overspreading saidoutput signal to form an overspread signal; and shaping said overspreadsignal to match power spectrum of the shaped signal to power transferfunction of said twisted pair wire.
 2. The method of claim 1 where saidincoming data arrives a single serial input.
 3. The method of claim 1where said incoming data arrives at a plurality of serial inputs, eachcarrying signal of one or more of said source.
 4. The method of claim 1where said incoming data is pulse amplitude modulated.
 5. The method ofclaim 1 where said overspreading in said step of modifying is performedby multiplying said output signal with a signal that has the value +1and −1.
 6. The method of claim 1 where said sources are taken from a setthat includes POTS signals, IDSN signals, and T1 signals.
 7. A methodfor developing a signal adapted for transmission over a twisted pairwire comprising the steps of: accepting an incoming plurality of flowsS_(i), i=1, 2, . . . M , each having a data rate R_(i); splitting asignal that is related to each flow S_(i) into a set of N_(i) parallelstreams, s_(ij), j=1, 2, . . . N_(i), such that each of the parallelstreams has a symbols rate of R_(s); relative to each of set i of N_(i)parallel streams, s_(ij), spreading each stream s_(ij), j=1,2, . . .N_(i), with orthogonal code W_(j) taken from a set of orthogonal codeW_(m), m=1,2, . . . N_(M), and summing the spread streams to form asummed output stream K_(i) that has a rate of R_(c); modifying outputstream K_(i) for at least some values of i by overspreading K_(i) toform an overspread signal; and adding the overspread streams K_(i) toform said signal adapted for transmission.
 8. The method of claim 7where said overspreading results in rate of overspread stream K_(i) tobe greater than rate of overspread stream K_(i+1).
 9. The method ofclaim 7 where said signal related to flow S_(i) is adaptively modulatedversion of S_(i).
 10. The method of claim 7 where said signal related toflow S_(i) is related to 2^(i−1)S_(i).